1) Field of the Invention
The present invention relates to a radio-controlled timepiece that receives a radio signal and corrects the time based on the time information in the radio signal. This invention also relates to method of receiving the radio signal and an electronic device that includes the radio-controlled timepiece.
2) Description of the Related Art
Radio-controlled timepieces have become popular in, for example, Germany, England, America, and Japan. A wide-band carrier signal of tens of kHz is employed to transmit radio signal that contains time information to the radio-controlled timepieces. Although the carrier signal in all these countries is a wide-band signal, the pulse waveforms for 0, 1, and the like, differs in each country.
A conventional radio-controlled timepiece has been described in the Japanese Patent Laid Open Publication No. 8-201546. This radio-controlled timepiece has a radio-controlled time correction function in which it receives a wide-band standard frequency signal (i.e., a radio signal) and corrects the time based on the time code (i.e., time information) in the standard frequency signal. This time correction function is activated at a predetermined time or when instructed by the user of the timepiece. Precisely, the radio-controlled timepiece has a receiver and a time unit and, the receiver receives the standard frequency signal when it receives a reception approval signal from the time unit.
The format of the time code transmitted in the standard frequency signal in Japan (Japanese standard frequency signal) is shown in FIG. 9. The time code is transmitted at a rate of one bit per second. Moreover, time code transmitted in one minute is considered as one frame. The information about minute, hour, days elapsed from January 1st, year, and day of a week are included in one frame.
Since the minute, hour etc. are determined based on the position of 0 second, it is necessary to decide the position of 0 second. A marker P code is also included, apart from 0 and 1. The waveform of 0, 1, and P are shown in FIGS. 10 to 12, respectively. The P code appears at many instances in a single frame. For instance, the P code appears at instances of 0 second, 9th second, 19th second, 29th second, 39th second, 49th second, and 59th second. Thus, the P code appears continuously only for two times, i.e., at 59th second of one frame and at 0 second of subsequent frame. In other words, when the P code appears consecutively twice it means that a new frame has started and, it is the 0 second position. Once the position of the 0 second is detected, the waveform (0, 1, or P) of the data is determined for each bit of data received at every second.
Conventionally, the waveform is determined as follows. That is, as shown in FIGS. 10 to 12, data is sampled at the two points, TA and TB, and the waveform is decided based on weather the sampled values indicate high or low. To be more specific, as shown in FIG. 10, when TA and TB are both high, it is 0 waveform. As shown in FIG. 11, when TA is high and TB is low, it is 1 waveform. As shown in FIG. 12, when TA and TB are both low, it is P waveform.
FIG. 13 shows the format of the data transmitted in the standard frequency signal in America (American standard frequency signal). The format of the data transmitted in the American and Japanese standard frequency signals is the same with regard to the items of minute, hour, days elapsed, and P code, but it is different for the item of year.
The waveforms of 0, 1, and P in the American standard frequency signal are shown in FIGS. 14 to 16. These waveforms differ from the waveforms of 0, 1, and P in the Japanese standard frequency signal. Nevertheless, in America, the determination of the waveform is performed in the same manner as in Japan, i.e., by sampling the waveform at the two points. To be more specific, as shown in FIG. 14, when TA and TB are both high, then it is 0 waveform. As shown in FIG. 15, when TA is low and TB is high, it is 1 waveform. As shown in FIG. 16, when TA and TB are both low, it is P waveform.
How the time code is extracted, after the determination of waveforms of 0, 1, and P, has been described in Japanese Patent Laid Open Publication No. 11-304973 filed by the applicant of this patent application.
In the conventional radio-controlled timepiece, the reception of the standard frequency signal is greatly affected by the electric field intensity and the signal-to-noise (S/N) ratio. The receiving unit in the radio-controlled timepiece can demodulate the radio signals and output waveforms that are substantially same as the waveforms shown in FIGS. 10 to 12 or FIGS. 14 to 16 when the electric field intensity is high and the S/N ratio is low. However, if the electric field intensity is low and the S/N ratio is high, the receiver outputs faulty waveforms. FIGS. 17 to 19 show examples of the faulty waveforms of 0, 1, and P, respectively. In FIG. 19, a reference numeral 1900 indicates spikes in the P waveform.
Although the waveform is faulty, sometimes it is not possible to decide that the waveform is faulty. To be more specific, since both the values will be low when sampling is performed at two places in the waveform shown in FIG. 18, although this waveform is the 1 waveform, it is wrongly identified as the P waveform and no error signal is output. In this case, since an error signal is not output, the reception process is continued as it is and, time is corrected based on wrong information.
In case of the waveform shown in FIG. 19, the waveform may be detected as a correct waveform or a faulty waveform depending on the sampling positions. If a faulty waveform is detected as a correct waveform, the reception process is continued as it is and, time is corrected based on wrong information. On the other hand, if a correct waveform is decided as a faulty waveform, the reception process is repeated from the beginning and, it takes time for completing the reception process. In addition, there is an unnecessary consumption of electricity when the reception process is repeated.